Process of preparing dense non-fibrous nitrocellulose



Oct. 9, 1962 J. E. LUFKIN 3,057,012

' PRocEss oF PREPARING DENsE NoN-FIBRoUs NITRocELLuLosE Filed May 27,1959 2 Sheets-#Sheet 1 FIG. I.

JAMES E. LUFKIN .Mum

ROLL NIP PRESSURE Il PSI.

OC 9, 1962 J. E. Lul-'KIN 3,057,012

PROCESS OF PREPARING DENSE NON-FIBROUS NITROCELLULOSE Filed May 27, 19592 Sheets-Sheet 2 I6 z DENSE N R0 cEL uLosE clon |4 l2 s l0 UD L 8 e 3 IBou NIIR CELL L0 E w 4 REGION :L 2

lo l2 I4 I6 la 20 22 24 2628 3052 54 36 5a 4o 42 44 BULK DSIITY LBS.DRY/cu. FT.

DENSE NITRO CELLULOSE REGION Fl ROUS NITRO ELLULOSE RE I0 INVENTOR JAMESE. LUFKIN WKK-Lm ATT NEY (WIDE OPEN) o 20o 40o 600 800 MEDIAN PARTICLESIZE-MICRONS United States The present invention relates to a method oftreating nitrocellulose to render it more safe for shipment and storage.More particularly, the present invention pertains to a method fortreating nitrocellulose which, in addition to making it safer, has thefurther advantages of greatly raising its bulk density, enhancing itsdissolution properties, and greatly improving its ability to flow freelyfrom containers in which it is shipped and/or stored.

The present invention is a continuation-in-part of my prior copendingapplication Serial No. 682,581, filed September 9, 1957, now abandoned,which, in turn, is a continuation-in-part of my prior copendingapplication Serial No. 607,255, filed August 31, 1956, now abandoned.

`Nitrocellulose is a cellulose derivative which has found extensive usein a Wide variety of industrie-s. It is prepared commercially by thedirect nitration of cellulose in any convenient form, such as purifiedwoodpulp or cotton linters. The nitration is usually performed with anacid mix consisting essentially of nitric acid, sulfuric acid, and waterin suitable proportions, although other nitrating media are sometimesused.

Nitrocellulose is rarely, if ever, shipped or stored in a dry formbecause of its greatly increased sensitivity to ignite when dry. Forthis reason, nitrocellulose is almost always maintained in a wet form.Where the presence of moisture can be tolerated in the end use, thenitrocellulose is wetted with water. Commercial water-wet nitrocellulosecharacteristically contains about Ztl-25% water. For some end uses,however, the presence of moisture in nitrocellulose is extremelyobjectionable and in such cases the nitrocellulose is wetted withalcohol; usually ethanol, isopropanol, or butanol. Commercialalcohol-wet nitrocellulose normally contains about 30 to 35% totalvolatiles; the latter being primarily alcohol with small amounts ofmoisture. Throughout this speciiication, the term wet nitrocellulose isintended to designate nitrocellulose which has been wetted with water,alcohol, or other suitable liquid.

The density of nitrocellulose which has not been compressed or compactedin any way is in the neighborhood of about l pounds per cubic foot (drybasis). For storage and shipment, this 'material is rammed intocylindrical drums to a density of 20-25 pounds per cubic foot. 'Ihecommercial nitrocellulose drum contains on the average of about 135 toabout 160 pounds of nitrocellulose (dry basis) per drum.

Though wetting the nitrocellulose, as described above, clearly minimizesthe hazards of storing and shipping nitrocellulose, some danger stillremains. For example, if an open standard commercial drum of alcohol-wetnitrocellulose should ignite, a violent eruption will ensue which willsend a ball of fire upwards for a distance of 25 to 50 feet and maypropel a shower of burning particles and sparks a considerable distancelaterally outward from the drum. In the event of accidental ignition,any personnel in the immediate vicinity are likely to be seriouslyinjured. In addition, the great shower of burning particles and sparksrepresents an extreme hazard in that other drums of nitrocellulose orother inflammable material in the area may also be ignited, leading toan even more widespread and dangerous conagration.

It is an object of the present invention to treat nitrolarent O3,657,012 Patented Oct. 9, 1962 cellulose in such a way as to greatlyminimize the effect of accidental ignition. It is a further object ofthe present invention to provide such a treatment which, in addition,will increase the bulk density of the nitrocellulose and enhance itsdissolution properties. It is a still further object of the presentinvention to provide such a treatment which is convenient andeconomical. Other and additional objects Will be readily apparent from aconsideration of the following specification and claims:

Broadly stated, my invention involves subjecting wet fibrousnitrocellulose to severe compressive pressures of a magnitudehereinafter specified in order to compact the wet nitrocellulose into acompact sheet or into a particu late form consisting ofirregularly-shaped, flat particles which may be thereafter broken upinto smaller particles by a mild granulating action if necessary. Thecompression may be performed in any suitable way, and the particularapparatus which is used forms no part of my invention.

For example, the wet fibrous nitrocellulose may be placed on a simpleroll mill of the type which is conventional in the rubber industry forcompounding rubber stocks prior to curing. Such a roll millcharacteristically consists of a pair of cooperating rollers spaced ashort distance apart and driven in opposite directions. One roller mightbe driven in a clockwise direction at a given speed, and the other mightbe driven in a counterclockwise direction at the same or a diiferentspeed, or the second roller may idle. The space between the rollers isnot generally critical insofar as my invention is concerned and may bevaried widely; the preferred setting in any instance depending upon thetype of nitrocellulose, the rate of nitrocellulose feed, and variousother factors. Similarly, the thickness of the nitrocellulose disk-likeparticles or sheet formed by the rollers may vary from exceedingly thinfilms a few thousandths of an inch thick to relatively thick particlesor sheets.

After the wet nitrocellulose has been compressed, as described above, itmay be crumbled by subjecting it to a mild granulating action. I use thelatter term in its broadest sense to include any mechanical working oragitation which tends to break up the flat particles or sheets into asmaller particulate form. In many cases the mere dropping of theparticles or sheet from the rollers to the surface on which the rollersare mounted is sufficient to crumble all or a significant proportion ofthe compressed nitrocellulose. Depending upon the thickness of thesheets or particles and the type of nitrocellulose, it may be desirableto subject them to -a mild tumbling or agitation `or to the action ofslowly rotating teeth to insure that the pressed nitrocellulose isreduced to a particulate form for packing into a drum. In thisconnection, any mild mechanical working is operable including, forexample, shaking, crumbling, pulverizing, vibrating, chewing,comminuting, tumbling or the like. In many cases, however, the flatdisk-like particles which result from the compression are only severalinches wide on the average and may themselves be loaded into a drumwithout rst breaking them up into still smaller particles.

The invention will be better understood from a consideration of theattached drawings in which FIGURE l represents an elevational schematicview of one form of apparatus suitable for carrying out the process ofinvention. FIGURES 2 and 3 show in graph form the relationship betweenthe pressure applied to the bulk density of the finished product, and tothe particle size of the nitrocellulose starting material, respectively.

In FIGURE l, 1 and 2 represent a pair of abutting rollers; roller 1idling on its shaft and roller 2 being driven in the direction indicatedby any suitable drive means (not shown). Roller 2 is journalled in theend of a pair of hydraulic ram arms 3 by means of which roller 2 may beforced to bear against roller 1 under great pressure. Beneath therollers 1, 2 is a shredder or comminuter containing two sets ofparallel, rotating, intermeshing teeth 6, 7.

The wet nitrocellulose from hopper 4 feeds into the roller set 1, 2 andis there severely compressed into a hard, dense, compact, non-fibrous,sheet-like form or the like, which, in turn, falls through the comminute5 Where the compressed material is broken up; the resultant particlesfalling for collection, for example, onto conveyor 8.

The process of the present invention is primarily applicable to thetreatment of so-called industrial nitrocellulose, i.e., conventionalfibrous nitrocellulose products having a nitrogen content of 10.8% to12.3% and used industrially for lacquers, coatings, plastics, and thelike, as distinct from guncotton and other nitrocellulose propellantproducts having a higher nitrogen content. The latter varieties ofnitrocellulose, having a nitrocellulose content greater than 12.3%nitrogen are usually referred to in the trade as military grade orpowder grade nitrocellulose. Though the invention may have somebeneficial effects in connection with treatment of military gradenitrocellulose, the greatest benefits are derived in the industrialnitrocellulose field and this represents by far the most significant andpreferred embodiment of this invention.

To ob-tain the principal advantages of the invention, as described moreparticularly hereinafter, it is essential that the compressive forces towhich the brous nitrocellulose is subjected be of a certain criticalminimum magnitude. This critical minimum pressure will vary somewhat ona case-to-case basis depending upon the nature of the nitrocellulosestarting material.

The graph which appears in FIGURE 2 depicts the variation in bulkdensity of the compressed nitrocellulose product as a function of thepressure applied to effect the compression. The pressure was applied byfeeding the nitrocellulose (alcohol-wet) continuously, at the rate of3,000 pounds/hour, through equipment of the type illustrated in FIGUREl. The rolls were metal, 15 inches in diameter and 39 inches long, andwere rotated at about 20 r.p.m. The family of curves shown in this graphrelate to three different nitrocellulose materials. Curve A representsan industrial grade of nitrocellulose known to those skilled in the artas 5-6 second regular soluble. It contains 12% N2 and has a steel ballviscosity of 5-6 seconds measured in 5012 solution at 25 C. `Curve Brepresents a 1/2 second regular soluble grade containing 12% N2 andhaving a steel ball viscosity of 3-4 seconds in 5020 solution at 25 C.Curve C represents a 1A second regular soluble grade containing 12% N2and having a steel ball viscosity of 2.5-4.0 seconds in 5025 solution at25 C.

It will be noted from FIGURE 2, that as the pressure on thenitrocellulose feed is increased from zero, the bulk density of theresultant product increases fairly rapidly until the pressure reachesthe vicinity of 6000-10,000 p.s.i. In this area, the slopes of thecurves reverse and the rate of increase in bulk density increases moregradually as the pressure rises. Somewhere in the 6000-10,000 p.s.i.range the nitrocellulose starting material undergoes a basic change incharacter. At some point above about 6000 p.s.i., the nitrocellulosebegins to lose its fibrous nature in favor of a hard compact essentiallynon-fibrous form, which I refer to as dense nitrocellulose in view ofits greatly increased density compared with the ordinary fibrousmaterial. This conversion from the conventional brous to the non-fibrousdense form, of course, does not occur suddenly, but rather occursgradually starting at some point above a pressure of about 6000 p.s.i.The conversion is generally complete for all industrial grades ofnitrocellulose when the applied pressure reaches about 10,000 p.s.i.,and at some point intermediate between these two pressures, thenitrocellulose may be considered to be essentially the dense producthaving little or no fibrous components.

Thus, in FIGURE 2, points 1, 2, and 3 on curves A, B, and C,respectively, designate the lowest pressures at which the nitrocellulosewas found to have passed from the primarily fibrous state into theessentially non-fibrous dense form. The line 1-23 connecting thesepoints divides the graph into two general zones; the area below the linerepresenting the conventional fibrous nitrocellulose region and the areaabove the line representing the non-fibrous dense nitrocellulose regionwith which the present invention is concerned.

One of the principal ways of characterizing nitrocellulose (aside fromN2 content) and differentiating one industrial grade from another is interms of inherent viscosity. The viscosity of different nitrocellulosegrades is directly dependent upon the degree of polymerization of thenitrocellulose. In commercial practice, a reduction in viscosity, i.e.,lowering the degree of polymerization, is accomplished by means of ahigh-temperature digestion. As is well-known to those skilled in theart, a reduction in degree of polymerization, by digestion or otherwise,is invariably accompanied by a corresponding reduction in the lengths ofthe individual fibers, i.e., fiber particle size. It is possible,therefore, to identify the nitrocellulose products represented by thefamily of curves in FIGURE 2 in terms of iiber particle size rather thanby specific nitrocellulose types. This is particularly convenientinasmuch as the pressure-density relationship shown in FIGURE 2 has beenfound to hold true for liber particle size variations even within asingle nitrocellulose species. That is to say, a family of curvessimilar to A, B, and C of FIGURE 2 can be drawn for three differentnitrocellulose starting materials which differ only in fiber particlesize and in no other way. Fiber lengths can be reduced substantially bypurely mechanical means, eg., in an attrition mill, without significanteffect on the degree of polymerization of the product.

The particle size distribution of fibers may be readily determined in anaccurate manner, for example, by means of a four-screen ClarkClassifier, manufactured by the Thwing-Albert Instrument Co. This latterinstrument is widely used and relied on in the paper and pulp industry.See A. E. Reed and I. dA. Clark, An Instrument for Rapid Fractionationof Pulp, TAPPI (published by the Technical Association of the Paper andPulp Industry), vol. 33, No. 6.

By means of a four-screen Clark Classifier, the median fiber size forthe three nitrocellulose products represented by curves A, B, and C inFIGURE 2 was determined to be 2000, 650 and 160 microns, respectively.The median fiber size represents that fiber length which is smaller thanthe half of the fibers in the lot classified and larger than theremaining half.

It is therefore possible to plot the minimum critical pressure requiredto produce an essentially non-fibrous dense nitrocellulose in accordancewith the invention as a function of median nitrocellulose particle size.Such a graph 1s depicted in FIGURE 3 for the range of median particlesizes encountered in ordinary industrial nitrocellulose products. Ateither end of the straight line shown, a complete plot would show acurve approaching a vertical asymptote, but this is of no realsignificance insofar as ordinary industrial nitrocellulose products areconcerned. With regard to the latter, the relationship is essentially alinear one. For ordinary industrial nitrocellulose products, i.e., thosehaving a nitrogen content of 10.8% to 12.3% and a median fiber length ofto 3000 microns, the minimum critical pressure required to convert theordinary fibrous material to an essentially non-fibrous dense productmay be taken as the slope of the line shown in FIGURE 3, or expressedmathematically as:

where P is pressure in pounds per square inch and M 1s the median fiberparticle size in microns. The

pressure P, determined in accordance with this relationship, representsthe critical minimum pressure which must be applied to the conventionalfibrous nitrocellulose composition in order to convert it to theimproved essentially non-fibrous, dense form having the many novelcharacteristics and advantages hereinafter described.

The invention is further illustrated =by the following examples.

Example 1 200 pounds per hour of alcohol-wet nitrocellulose (12%N)containing 19% isopropyl alcohol and having a median liber particle sizeof 650 microns were fed continuously to a pair of cooperating metalrollers 6 inches in diameter and l2 inches in length. One of the rollerswas rotated in a clockwise direction at about 20 r.p.m. and the otherroller was rotated in a counter-clockwise direction at about 28 r.p.m.The peripheries of the rollers were spaced about 0.015 inch apart. Theforce applied to the rollers was such that the pressure on thenitrocellulose in the nip was about 17,000 p.s.i. The compressed wetnitrocellulose disk-like sections which emerged from the rollers werepermitted to fall 8 inches to the oorpan beneath the rollers where asubstantial proportion of them crumbled due to the impact of the fall.

Example 2 Approximately 3000 pounds per hour of alcohol-wet 5-6 secondregular soluble nitrocellulose (12% N2, containing 21% ethyl alcohol)having la median liber particle size of about 2000 microns, were fedcontinuously to a pair of cooperating metal rolls inches in diameter and39 inches in length. One of the rolls was rotated in a clockwisedirection at about r.p.m. and the other was rotated in acounter-clockwise direction at approximately 20 r.p.m. A total pressureof about 50 tons was applied by means of hydraulic cylinders bearingupon both ends of one roll; the second roll being lixed. This totalpressure was equivalent to a pressure on the nitrocellulose of about16,500 p.s.i. The dense, compact, compressed wet nitrocellulosedisk-like sections which emerged from the rolls were permitted to fall10 inches into a cutting device, comprised of a horizontal rotatingshaft (150 r.p.m.) and a stationary Shaft, each iitted with a series ofintermeshing T-shaped blades. The final products consisted of iiakesaveraging about 1 to 2 in.2 in area and 0.040" thick. The free-flowingproduct was readily packed into `a standard ICC-6J shipping container at200 lbs. dry nitrocellulose and adjusted to a iinal alcohol content ofExample 3 Approximately 3000 pounds per hour of water-wet nitrocellulose(11.6% N2), containing 23% water, were fed continuously to the same pairof cooperating metal rollers utilized in Example l. The roller speedsand loading pressure, as well as the method of disintegrating thedisk-like sections produced were identical to those specilied inExample 1. The final water-wet, dense nitrocellulose product consistedof akes averaging about l to 2 in.2 in area and 0.040 thick. Thefree-iiowing product was readily packed into a standard ICC-6J shippingcontainer at 200 lbs. dry nitrocellulose and adjusted to a iinal watercontent of 23%.

Example 4 Approximately 3000 pounds per hour of alcohol-wet, one-halfsecond regular soluble nitrocellulose (12% N2, containing 21% ethylalcohol), having a median fiber particle size of about 650 microns, werefed continuously to the same pair of cooperating metal rollers utilizedin Example 1. The roller speeds and loading pressure, as well as themethod of disintegrating the disk-like sections, were identical to thosespecified in Example 1. The rfinal alcohol-wet, dense nitrocelluloseproduct consisted 5 of flakes averaging about l to 2 in.2 in area and0.040" thick. The free-flowing product was readily packed into astandard ICC-6J shipping container at 240 lbs. dry nitrocellulose andadjusted to a final alcohol content of 25%.

The tests I have performed indicate quite conclusively thatnitrocellulose which has been treated in acc-ordance with the presentinvention is much safer to ship and to store than is the conventionalfibrous nitrocellulose currently available from commercial sources, asis illustrated by the following examples.

Example 5 An ICC-6] galvanized steel drum (inner diameter 221/2 inches,inner height 33% inches) was filled to the 50% level with ordinarycommercial, isopropanol-wet, librouse nitrocellulose (12% nitrogen, 30%total volatiles). A second identical drum was iilled to the 50% levelwith the treated nitrocellulose material of Example l adjusted to a 30%total volatiles content. Both drums were ignited simultaneously withseparate squibs. The results of both ignitions are indicated by thefollowing table:

The procedure of Example 5 was repeated with both the regular andtreated nitrocellulose being first adjusted to a 25% total volatilescontent. The results of both ignitions are recorded in the followingtable:

Time after ignition (min.) Regular un- Treated material treated materialof Example 1 lfm mild name mild name. Do. Do. Do. Do. D

small flare-up.

10 do Do. 12 (flame extinguished With Water no nitrocellulose a littlenitrocellufrom rehose). left. lose left in bottom.

Example 7 The procedure of Example 5 was repeated with the treatednitrocellulose being first adjusted to a 20% total volatiles content.The results of both ignitions are recorded in the following table:

Time after ignition (min.) Regular untreated Treated material ofmaterial Example 1 liz mild ame mild flame.

V start eruption Do.

1 end eruption.. Do.

2 mild flame small flare-up.

t e gu ed with alittle nitrocellulose of the orlginal Water from rehose)left in bottom. nitrocellulose u nconsumed.

ln Examples 5-7, the term mild flame refers to a low quiet flameextending no more than about 1-2 feet into the air above the top surfaceof the nitrocellulose. By small are-up is means a modest flame of somevigor extending 5-10 feet in the air. The term eruption designates avigorous and extremely active flame shooting upwards a distance of 25-50feet in the air and propelling a shower of burning particles and sparkslaterally outward for a considerable distance, 20-40 feet.

It will be readily apparent from the foregoing that in the event of anaccidental or spontaneous ignition, drums containing applicants treatednitrocellulose are considerably safer than drums containing ordinarycommercial material. Upon ignition, the latter burns with a vigor andintensity which in most instances is many times greater than that of thetreated material. A further factor of equal importance is that theregular material ares up much more quickly than does the treatedmaterial. Upon ignition, the regular material erupts almostinstantaneously giving little or no time for personnel in the area toescape or take steps to extinguish the flame. The treated material, onthe other hand, either does not erupt at all or flares up only after atime lag sufficiently long to permit personnel to stand clear or toextinguish the blaze.

Numerous advantages accrue from the above-described burning propertiesof nitrocelulose treated in accordance with the present invention. Thetreated material is, of course, much safer in the event of ignition withrespect to personnel and property in the vicinity since the blaze ismuch less severe and more easily extinguished if it Should occur. Inview of these improved safety characteristics, it may very well bepossible to safely reduce the total volatiles content, i.e., the alcoholwith which the nitrocellulose is wetted, thereby effecting a substantialsavings in the alcohol and in freight costs.

In addition to the safety and attendant advantages which are achieved bymeans of the present invention, several other incidental but extremelyimportant advantages also result. For one thing, the bulk density of thetreated material is much higher than that of the ordinary material. Itis thus possible to pack the customary contents of a commercialnitrocellulose drum (about 230 lbs. of material) in a container which is25-30% smaller in volume. On the other hand, if the container is notreduced in size, it is now possible to pack the container with up to25-30% more nitrocellulose than has heretofore been possible. From thestandpoint of economy in shipment and storage, this is obviously anextremely valuable achievement. The following table illustrates thedifferences in bulk densities between several types of regularcommercially available nitrocellulose products before and after theyhave been treated in accordance with the process of the presentinvention:

Bulk Density (Dry Basis) Total Nitrogen, Wetting Agent Volatiles,

Percent Percent Regular Treated Product Product (lbs/cu. ft.) (lbs/cu.it.)

12.0 Isopropanol.-. 19 9. 1 16. 6 12.0--- do 20 14.2 24.0 21 6. 6 12. 16 23 9. 8 14. 0

In addition to a marked increase in the bulk density of thenitrocellulose, the process of the present invention also enhances theability of the nitrocellulose product to enter into solution, as isindicated by the following example.

Example 8 30 seconds with a single-paddle stirrer, one inch from thebottom operating at 300 rpm. 989 grams of toluene was then added to eachbeaker and the contents of the beakers were then agitated for anadditional 30 seconds with the stirrer. Thereafter, 359 grams of 88%ethyl acetate was added to each beaker and the agitator was turned onagain. The agitator was permitted to run continuously except that it wasstopped every l5 minutes to check the solution until the nitrocellulosewas completely dissolved. On this basis, the regular commercial materialwas found to completely dissolve in 21/2 hours whereas the materialwhich had been treated in accordance with the process of the presentinvention dissolved in a period of only 11/2 hours.

In addition to all of the foregoing advantages of the present process,nitrocellulose which has been treated in accordance with the presentinvention has the still further advantage of remaining free-liowable atall times even when stored in drums for extended periods. Regularcommercial material when placed in a container invariably forms a mattedmass known as hard-pack, and the resistance of this hard-pack to flowhas plagued nitrocellulose consumers for many years. In order to empty acommercial container of ordinary wet nitrocellulose, it is necessary forthe operator to tilt the container and to extract the material manuallywith a pitchfork or similar implement or to use some other mechanicalaid to break the hard-pack and dislodge the nitrocellulose so that itwill flow from the container. 'I'his is an inconvenient andmoney-wasting operation from the point of view of most industrialnitrocellulose consumers and one which is completely eliminated by thepresent invention. Nitrocellulose which has been treated in accordancewith the present process will not form hardpack and will flow freely andquickly from any container in which it has been stored, even for longperiods.

When treating nitrocellulose in accordance with the present invention,any suitable means for compressing the wet nitrocellulose into compactsheets or into flat, disk-like particles may be used in lieu of a rollmill. For example, intermittent ramming may be used, or a single heavyroller on a ilat surface. It will be readily apparent, however, that thecontinuous feature of a roll mill offers attractive economic advantagesover other suitable methods, though the invention is by no means limitedto this particular type of apparatus.

According to the present invention an essentially nonbrous densenitrocellulose product having all of the various advantages and improvedproperties described above and may be prepared by vsubjecting aconventional fibrous nitrocellulose product to compressive forces of aminimum magnitude defined by the following equation:

wherein P equals the pressure in pounds per square inch and M equals themedian particle size in microns, and thereafter mechanically Working orbreaking up the resultant compressed product. It is vital, however, thatthe pressure applied be not less than the amount specified, asdetermined by the above-named relationship, if the improved propertiesand advantages of the present invention are to be obtained. I have foundthat the objectives of the present invention cannot be achieved or theprincipal advantages realized if the pressure to which thenitrocellulose feed is subjected in the course of the compression is notat least as great as the minimum specified. Lower pressures, thoughsometimes effecting a slight improvement in some of the properties of anitrocellulose, will not accomplish the improvements described above,especially the improved safety characteristics, dissolution, and Howproperties, and the increased density. The lower pressures, short of theminimum specified, simply compact the nitrocellulose into large, thick,irregularly shaped plates which tend either to be converted `back totheir original fibrous condition or to be broken into hard solid lumpswhen subjected to the use of a mechanical shredder.

As noted above, the pressure may be applied to the nitrocellulose in anysuitable manner as, for example, by a pair of cooperating rollers, asingle roll and plate, intermittent ramming, or the like. A pair ofcooperating rollers arranged as illustrated in FIGURE 1 represents amost convenient way of carrying out the invention, and therefore,represents the preferred embodiment of the invention. Generallyspeaking, any pressure greater than that indicated above as the minimumrequirement is suitable. I have found, however. that pressures in thevicinity of 15,000 p.s.i. or higher, which is substantially above theminimum required, yield consistently fine results inasmuch as suchpressures consistently serve to convert the conventional fibrousnitrocellulose into a compact, dense, completely free-flowing productexhibiting all the properties described above. For most industrialnitrocellulose products, therefore, I prefer to operate in the range of15,000 to 17,000 p.s.i. though lower pressures Within the limits definedabove are definitely operable. The only upper limitation on the amountof pressure which may be employed is the pressure at which thenitrocellulose feed begins to char. As a practical matter, however,economic considerations would dictate that pressures above the preferred15,000 to 17,000 p.s.i. range would rarely be used since they offer noparticular advantages.

Throughout the specification, pressures to which the fibrousnitrocellulose is subjected are always referred to in pounds per squareinch. It must be borne in mind that the precise pressures pertinent inany case may vary slightly from those figures mentioned depending on thesize, efficiency, and surface conditions of the particular rollers orother equipment utilized to practice the invention. To some extent, eventhe age of the equipment will be a slight factor affecting the pressureemployed. For example, the spacing between the rollers shown in theattached FIGURE 1 on one occasion was nil prior to the start of thenitrocellulose feed, but after the rollers were in operation for sometime a spacing of about 0.040 inch Was noted when the feed was stoppeddue t flexing of the rollers and compression in the hydraulic loadingsystem. These individual equipment characteristics will,

therefore, affect slightly the actual pressures required in any singleinstance, but the dierences from the figures mentioned will never bevery great, running in the magnitude of a few percent at most from caseto case.

The shape or dimensions of the compressed nitrocellulose sections whichresult from the compression step is not critical to the invention. Thenitrocellulose may be pressed into the form of relatively longcontinuous sheets or it may be pressed into numerous relatively small (afew inches), individual, flat, irregularly-Shaped, disk-like particles.The latter are more likely to result if a roll mill is used andrepresents the preferred embodiment of the invention since it requires aminimum, if any, of subsequent mechanical working to break up thecompressed nitrocellulose.

Having thus described my invention, I intend to be limited only by thefollowing claim:

A method for improving the safety characteristics and dissolution and owproperties of Wet, fibrous, industrialgrade nitrocellulose having anitrogen content of about from 10.8 to 12.3% by weight which consists ofsubjecting the wet, fibrous, industrial-grade nitrocellulose to severecompressive forces which are not less than that determined by therelationship:

wherein P equals pressure in pounds per square inch and M equals themedian fiber particle size of the nitrocellulose starting material inmicrons, and thereafter mechanically breaking the resulting product intoparticles of smaller size.

References Cited in the file 0f this patent UNITED STATES PATENTS1,896,642 ONeil Feb. 7, 1933 1,978,071 York Oct. 23, 1934 2,210,871Boddicker Aug. 6, 1940 OTHER REFERENCES Bridgeman: The Compression of 46Substances to 50,000 Kg./Cm.2, Am. Acad. of Arts & Science, vol. 47, No.3, October 1940.

